This invention relates to a fastener which includes an insert made of a metal such as steel and having a head-retaining portion, a threaded shank and a flange between the two. The fastener also includes a cap encapsulating at least the head-retaining portion of the insert and made of a non-ferrous material such as zinc, aluminum or plastic. Thus, the part of the fastener which is covered by the cap is corrosion resistant. The head-retaining portion of the insert is polygonal in cross-section and the cap fits closely around this and is formed with surfaces which may be engaged by an ordinary wrench. Thus, when the cap is turned, the insert also is turned to drive the threaded shank into the work. In use, a resiliently flexible sealing washer is disposed beneath the flange and, when the fastener is tightened, the washer is flattened against the work to form a seal around the shank.